A typical x-ray tube comprises an evacuated glass tube with an anode and cathode spaced relative to each other within the tube. The anode and cathode are maintained at a high differential voltage relative to each other, typically on the order of about 150 kV or less for medical applications. The anode may be maintained at ground, and the cathode may be maintained at a relatively high negative potential, e.g., -150 kV. Alternatively, the anode may be maintained at a positive potential, e.g., +75 kV, and the cathode may be maintained at a negative potential, e.g., -75 kV. The cathode thermionically emits electrons which are electrostatically directed onto a target surface of the anode with sufficient energy to generate x-rays which emerge from the target in a diffuse pattern. A considerable amount of heat is generated at the anode during operation of an x-ray tube. Thus, in x-ray tubes having stationary anodes, a cooling fluid, such as oil, typically flows through a base portion of the anode to cool the anode to permit a higher x-ray output and prevent overheating and deformation of the target surface. In rotating anode x-ray tubes, on the other hand, the target surface is typically defined by a rotating disc so that the region of electron incidence is distributed over an annular target surface area. Thus, in rotating anode x-ray tubes, the energy of the incident electrons is typically distributed over a larger surface area than in stationary anode tubes, thereby allowing for higher peak energies operating for short times.
The cathode comprises one or more filaments for generating the electron beam, and the filaments project a focal spot area onto the target surface of the anode. Typically, the focal spot area is rectangular; however, the cathode filaments may take any of various different shapes, and thus the focal spots likewise may correspondingly vary in shape. The x-ray tube is mounted within a hermetically-sealed housing, and the housing defines an x-ray port radially aligned with the focal spot on the target surface. The housing is filled with oil to electrically insulate the tube, and frequently, a heat exchanger is either mounted to the housing, or remotely connected to the housing to cool the oil and thereby cool the x-ray tube. The housing is typically formed of aluminum, and the interior surfaces of the housing are lined with a radiation absorbing material, typically lead. An x-ray window made of an x-ray transmissive material is mounted over the x-ray port to allow the diffuse radiation beam to pass out of the housing through the x-ray window only. Frequently, the transmissive window is made of a transparent, polymeric material, such as polycarbonate, and defines a frusto-conical or cup-like shape. The cup-like window projects into the housing, such that the base of the window is spaced closely adjacent to the exterior surface of the glass x-ray tube. Alternatively, the x-ray windows have been made of non-transparent, metallic materials, such as aluminum and beryllium, which are also radiation transmissive. However, the polymeric windows may be made transparent, and thereby allow an operator to view the interior of the housing. In addition, the polymeric windows are non-conductive, and therefore may be mounted in close proximity to the glass x-ray tube. The metallic windows, on the other hand, must be spaced a sufficient distance from the glass x-ray tube to avoid high voltage arcing between the metallic window and tube. The oil within the housing may include gas bubbles, particulate matter, or other non-homogeneous materials, and if any such bubbles or particulates pass between the tube and x-ray window during operation, they may show up as artifacts on the x-ray image. Accordingly, one advantage of the polymeric x-ray windows is that they may be mounted in close proximity to the tube in order reduce the thickness of oil between the tube and window and thereby minimize the possibility of any gas bubbles negatively effecting the x-ray images. One of the drawbacks of the polymeric windows, however, is that the high energy x-rays tend to destroy the molecular bonds of the polymeric material, and if such windows are not replaced, they can eventually craze and/or crack. This condition not only may negatively affect the x-ray images, but may destroy the hermetic seal of the housing and, in some cases, allow oil to leak from the housing.
It is well understood in the prior art, and particularly in connection with computerized tomography ("CT"), that the x-rays emanating from the focal spot should be precisely collimated into a fan or other preselected shaped beam, in order to cover the image detector surface. One phenomena that can have a significantly negative effect on the image quality of x-ray images is "off-focus" radiation (also referred to as "off-focal" radiation). Off-focus radiation is primarily produced by energetic back-scattered electrons that produce x-radiation outside of the focal spot. These secondary electrons tend to cause x-rays to be generated from broad areas of the anode and possibly surrounding material that may be at a positive potential relative to the cathode. In addition, high field emission electrons from portions of the cathode other than the filament may impinge on the target and possibly other surrounding material outside of the focal spot and, in turn, create additional off-focus radiation. Both standard radiographic and CT apparatus typically require a well-defined x-ray source. Accordingly, off-focus radiation reduces the image resolution of both conventional x-ray and CT imaging apparatus, and increases the radiation exposure level to patients and technicians.
In order to reduce off-focus radiation, and control the size and shape of the x-ray beam, conventional x-ray tube assemblies have incorporated a beam limiting device in the form of a lead plate mounted on the outer side of the x-ray window. The lead plate has formed therein a beam-defining aperture corresponding in size and shape to the desired beam. Thus, the lead plate operates as a first stage of control to define the size and shape of the beam and block any off-focus radiation outside the periphery of the aperture. One drawback associated with these types of prior art beam limiting devices, is that the lead is conductive, and therefore the beam limiting device cannot be located in close proximity to the high voltage x-ray tube. As a result, substantial off-focus radiation may be allowed to pass through the aperture and, in turn, degrade the resolution of the x-ray image.
Other prior art x-ray tube assemblies have included beam limiting devices mounted directly onto the glass x-ray tubes. For example, one such prior art beam limiting device is an elongated, arcuate-shaped device having a rectangular beam slot formed therethrough. The device is made of a radiation absorbing, non-conductive, thermoset material sold under the trademark LITHARGE.TM.. This beam limiting device is mounted directly onto the glass x-ray tube between the focal spot and the x-ray window, and is attached to the tube with a silicone ("RTV") adhesive. Thus, the rectangular slot is designed to control the size and shape of the x-ray beam, and the surrounding LITHARGE.TM. material is designed to filter out any off-focus radiation impinging thereon. One of the drawbacks associated with these types of prior art beam limiting devices is that they must be adhesively or mechanically attached to the glass tubes. The silicone adhesives, such as the RTV adhesives, tend to degrade over time, particularly as a result of radiation exposure. Accordingly, these types of beam limiting devices can become detached from the x-ray tubes and/or the silicone or other adhesives can break off or dissolve into the oil, which, in turn, negatively affects the dielectric and/or other properties of the oil. Another drawback of these types of beam limiting devices is that the thermoset materials require relatively expensive tooling, and do not lend themselves to allowing easy and inexpensive manufacture of complex parts.
Another drawback of the above-described prior art x-ray tube assemblies is that the use of lead to line the interior of the housings involves relatively time-consuming, labor-intensive, and expensive manufacturing procedures. Typically, the lead lining consists of a plurality of lead pre-forms, each of which must be cut and shaped with special dies and tooling to conform to a respective portion of the housing. Then, the pre-forms must be pressed into the housing, and fixedly secured to the housing. The lead is hazardous to handle, and therefore requires either expensive automated assembly equipment, and/or sophisticated procedures and handling equipment to prevent assembly workers from improper exposure to the lead. In addition, the lead presents significant problems and costs in connection with its disposal.
Accordingly, it is an object of the present invention to overcome one or more of the above-described drawbacks and disadvantages of prior art x-ray tube assemblies.